Slush hydrogen comprises a mixture of liquid hydrogen and solid hydrogen at hydrogen's triple point of 13.8.degree. K. and 52.8 torr. Slush hydrogen has significant advantages to Normal Boiling Point (NBP) hydrogen, which refers to liquid hydrogen having a boiling point of 20.2.degree. K. at 760 torr, these advantages being increased density and increased heat absorption capacity before vaporization relative to NBP liquid hydrogen. Similarly to NBP liquid hydrogen, slush hydrogen has good flow properties which makes it easily transportable. These advantages make it particularly suitable as a fuel source and coolant in aircraft.
Processes have been developed to produce slush hydrogen and some are referred to as the Auger-type, magnetic refrigeration, freeze-thaw, nozzle expansion and pumping-down. By and large each of these processes has been a batch process and there have been few reported cases of a continuous process for producing slush hydrogen by any of the above processes.
Representative prior art for producing slush hydrogen is noted in R. Schrawer, Production and Transport of Hydrogen Slush Research Report T 75-22 (1974) published for the Federal Ministry for Research and Technology (West Germany). The process most favorably employed was based on a combination of processes and the process incorporated the pumping-down process and the nozzle expansion process. In that process liquid hydrogen at its normal boiling point at ambient pressure is transported through a siphon line into a nozzle, expanded and injected into a vessel. Solid hydrogen is formed at the nozzle and is dispersed into the vessel. A vacuum pump is used to reduce the pressure in the vessel and draw off incoming saturated vapor thereby causing additional cooling and solid hydrogen formation. Schrawer points out that the pumping-down process is inherently reversible and supplies a higher slush yield than the irreversible nozzle process which incorporates isenthalpic throttling. The advantage of isenthalpic throttling is that the process lends itself to continuous slush production although no process was disclosed.
U.S. Pat. No. 4,009,013 discloses a variation in the processes described in the technical report above and pertains to the production of fine grained slush. In preparing slush having good transportation properties, solid particles in the slush are kept as small as possible. One problem associated with the pumping-down or pumping-off process is that solids develop which have a surface crystalline structure. To obtain finely grained slush for transportation it is necessary to destroy the crystalline surface solid, this usually being done by stirring. A finely grained slush is obtained in accordance with the patent by expanding high pressure liquefied gas through a nozzle into a chamber ultimately to a pressure below the pressure of the triple point in the gas-solid range and then to a pressure above the pressure of the triple point in the gas-liquid range. The cycling of pressure causes intermittent freezing and thawing of the surface hydrogen. Intermittent freezing coupled with agitation results in finely grained slush production.